Throughout the world there exist numerous subterranean tar sand formations containing high-density, high-viscosity bitumens which resist recovery by conventional means. The vast Athabasca tar sand field in Alberta Province, Canada represents one of the most notable examples of such formations. The Cold Lake deposits in the same province represent a similar formation.
A variety of methods have been proposed for improving the production of hydrocarbons from these formations by increasing their mobility, including both solvent injection and thermal steam stimulation processes. One currently utilized method is the cyclical steam stimulation ("CSS") process. In this method, an injection-production well is sunk into the bitumen-bearing formation and completed at a given depth, usually by perforation, so as to establish fluid communication between the well and the formation. Steam is injected through the injection-production well into the formation to mobilize the bitumens. The injection pressure is usually maintained above a threshold value corresponding to that required to maintain formation fracturing and parting to sustain practical injection rates. Steam injection is subsequently terminated and, often following some shut-in period, hydrocarbon containing fluids are produced from the same well.
In the course of successive CSS cycles, the reservoir in the vicinity of the injection-production well becomes depleted and a vapor zone forms about the well during the depressurization phase of each cycle. This is because CSS standard operating practice is to flow back or pump off from the formation a greater volume of fluids than the cold water liquid equivalent volume of steam which as been injected; the "voids" thus created fill with steam vapor or solution gas. During subsequent CSS cycles, bitumens which have been mobilized by the steam, which exist mostly in a liquid phase, move back downward through the vapor zone to the completion level of the injection-production well, where they enter the well via the perforations and are produced from the reservoir. Conversely, the injected steam moves through this vapor zone in an upward direction from the completion level of the injection-production well, due to the difference in densities between steam or vapor and formation liquids. The CSS process thus tends to mobilize and recover bitumens from an area of the reservoir which extends upwardly from the completion level of the injection-production well. Accordingly. CSS injection-production wells are typically completed near the lower oil horizon in the reservoir in order to maximize the portion of the reservoir which can be exploited by each well.
As described, the injected steam tends to move in an upward direction through the vapor zone; simultaneously, its thermal energy tends to diffuse outwardly into the reservoir. Due to this combined upward and outward spread of stimulating effects, the CSS process tends to mobilize and drain bitumens from a region of the reservoir which has a progressively greater horizontal cross-section at progressively higher levels in the reservoir. Each such region thus tends to be hemi-ellipsoidal, or bowl-shaped, in form, having a lower boundary (with respect to the portion of the reservoir which has not been mobilized) which slopes generally upward with a substantially positive gradient in an outward direction from the CSS injection-production well. In some reservoirs, the drained region may be lens-shaped or some other form having a lower boundary with a substantially positive gradient outwardly from an injection-production well, depending on formation characteristics and CSS injection techniques. "Substantially positive gradient" as used herein means a generally upward rate of inclination from a particular horizontal direction.
The drained region will have increased effective permeability relative to the non-mobilized portion of the reservoir due to the reduced presence of high-density, high-viscosity bitumens therein. As used herein, "increased effective permeability" refers to increased effective relative permeability to the gas phase. Gas-phase injection fluids, including steam in particular, will consequently flow much more readily through the region of increased effective permeability than through the remainder of the reservoir. As a result, heat penetration and additional bitumen mobilization during subsequent CSS cycles will take effect primarily at the boundaries of the bowl-shaped region.
In order to drain a larger area of a tar sand bed or similar field, a plurality of spaced-apart CSS injection-production wells may be used together in a coordinated pattern. The injection-production wells may be arranged in rows, clusters, or any of a variety of patterns known to those skilled in the art and selected to drain a particular reservoir. Because each of these CSS injection-production wells tends to mobilize and drain bitumens from a bowl-shaped region which has a progressively larger horizontal cross-section at progressively higher levels in the reservoir, the resulting regions of increased effective permeability about the injection-production wells eventually tend to override and intersect each other at upper elevations in the reservoir. Areas of communication are thus formed between the regions of increased effective permeability at upper elevations in the reservoir while non-mobilized regions of the reservoir still exist between the wells in the lower elevations. These non-mobilized regions are known to those skilled in the art as "cold-humps" and represent bodies of unrecovered hydrocarbons remaining in the reservoir. For reasons discussed below in connection with the description of the present invention, these cold humps resist recovery by further conventional cyclical steam stimulation via the injection-production wells, which renders the CSS process less economical in its later stages.
Although the preceding discussion has described the results of a typical CSS process, when applied to recovery of bitumens from a representative tar sand bed, those skilled in the art will recognize that there exist other methods of primary recovery of bitumens, such as conventional steam drive or solvent injection processes, which may result in similar conditions in a reservoir. It will be understood that the method of the present invention will be applicable to the recovery of bitumens from reservoirs having such similar conditions as a result of such other primary recovery methods, as well as to those wherein such conditions are the result of a CSS recovery process.
Still other methods which have been proposed for improving the production of hydrocarbons from tar sand beds and like formations include those which utilize in-situ combustion processes. In a typical in-situ combustion process, combustion is initiated or ignited in the cold reservoir and a combustion-sustaining fluid, typically air, is supplied to the combustion zone so as to expand the combustion front outwardly into the reservoir, thereby mobilizing the heavy bitumens.
One example of an in-situ combustion process is that disclosed in U.S. Pat. No. 3,441,083 (Fitzgerald), issued Apr. 29, 1969. Forward steam injection is first performed through the bottom portion of the formation from steam injection wells to spaced-apart production wells. In-situ combustion is then started in the top portions of the reservoir and sustained by air injected downwardly into the formation through injection wells. The heat and pressure created by the in-situ combustion, in combination with the heat and fluid flow provided by the steam injection, gravity, and thermal expansion, are intended to cause the hydrocarbons in the reservoir to flow downwardly in the reservoir, where they are then carried along to the production wells by the forward injection from the steam injection wells.
Although some similar processes utilizing in-situ combustion drive have, to a certain extent, proven workable in the recovery of bitumens from tar sand beds and similar formations, the low injectivity into the cold reservoir regions due to very low effective permeability of the formation results in a natural tendency for the combustion front, or "burn," to channel through the formation between the injection and production wells instead of evenly expanding through the formation. This channelling frequently leads to air or oxygen breakthrough and fingering of the burn, and consequently results in inefficient oxygen utilization and ineffective recovery of bitumens from the reservoir.
Consequently, there is still needed an efficient method for recovering bitumens from reservoirs containing heavy bitumens. Such a method would improve the total percentage recovery of bitumens from tar sand beds and like formations and would make it much more economical to produce hydrocarbons from such formations.